1. Technical Field
This application is related to methods of controlling the amount, shape and dimension of nanostructures by solution-based synthesis, more specifically, to the controlled formation of metal nanowires.
2. Description of the Related Art
Nano-sized materials (or nanomaterials) can differ markedly from their analogous bulk materials. In particular, the physical and chemical properties of nanomaterials correlate strongly with their size, shape and morphology. As a result, materials scientists have focused their efforts on developing simple and effective methods for fabricating nanomaterials with controllable shapes, sizes and morphology, hence tailoring their properties.
A solution-based synthesis (also referred to as the “polyol” process) is reasonably effective in large-scale production of metal nanostructures. See, e.g., Sun, Y. et al., (2002) Science, 298, 2176; Sun, Y. et al., (2002) Nano Lett. 2, 165; Sun, Y. et al., (2002) Adv. Mater. 14, 833; Kim, F. et al., (2004) Angew. Chem. Int. Ed. 116, 3759; and U.S. Published Application 2005/0056118. The polyol process involves the reduction of a precursor (e.g., a metal salt) of the metal nanostructure by a polyol (e.g., ethylene glycol) in the presence of poly(vinyl pyrrolidone) (“PVP”). Typically, the reduction is carried out at a temperature of no more than 200° C. Ethylene glycol serves the dual functions as the solvent and the reducing agent. Typically, the shapes and sizes of the nanostructures formed are affected by parameters including, relative amounts of PVP and the metal salt, concentrations of PVP and the metal salt, reaction time, and reaction temperature.
According to this method, metal nanostructures of various morphologies, including nanocubes, nanowires, nanopyramides and multiply twinned particles, have been obtained. However, a common problem of the polyol process is that a mixture of nanostructures of several shapes is produced. The problem is compounded by poor reproducibility of the process, which is likely caused by trace amount of contaminants in the constituents of the synthesis. See, e.g., Wiley, B. et al., (2004) Nano Lett. 4(9), 1733-1739.
The polyol process has been modified in efforts to form nanostructures of uniform shapes and sizes. For example, adding trace amount of chloride to the “polyol” process created single crystal truncated cubes and tetrahedrons. See, e.g., Wiley, B. et al., (2004) Nano Lett. 4(9), 1733-1739. Using a seed material (e.g., platinum or silver nanoparticles) in the “polyol” process generated uniform silver nanowires of high aspect ratio (about 1000). Sun, Y. et al. (2002) Chem. Mater. 14, 4736-4745. However, the problem of producing mixed nanostructures (e.g., the silver nanowires were accompanied by the formation of silver nanoparticles) persists in the existing methods. Additional separation steps are therefore necessary to obtain monodispersed nanostructures.
There remains a need in the art to fabricate nanostructures with controllable shapes and sizes in an effective and reproducible fashion.